The present invention relates to control systems for sputter coaters. More particularly, the present invention relates to control systems for reactive sputter deposition systems.
Reactive sputter deposition is used to form layers such as titanium dioxide, silicon dioxide, tin oxide, zinc oxide, indium-tin oxide, silicon nitride, and titanium nitride. These layers are useful for a variety of applications, including optical coatings and semiconductor integrated circuits. In reactive gas sputter deposition systems, material is sputtered off of a target in a reactive gas environment. The reactive gas is oxygen to form oxides or nitrogen to form nitrides. These compound materials form layers on a substrate.
Reactive sputtering tends to have a low deposition rate compared to sputtering in a non-reactive environment. The low deposition rate for reactive sputtering is due to the target becoming coated with the formed compound as the gas reacts with the atoms on the target surface. The oxide or nitride coating on the target has stronger bonds to itself and the target material than the material of the target has to itself. Thus, more energy is required to break those bonds and sputter the dielectric material off of the target. For a given plasma power, fewer compound bonds will break than metal bonds; therefore, the deposition rate of the compound will be lower than that of the metal target.
Furthermore, the secondary electron emission coefficient of most oxides is higher than the corresponding metal. This also reduces the sputter rate of oxides. For example, the sputter rate of pure titanium is ten times faster than the sputter rate of titanium oxide.
The remainder of this discussion will focus upon the sputter deposition of titanium dioxide, although similar issues are involved with the sputter coating of other oxides or nitrides. When the target surface is free from the oxide coating, the sputter rate is high. An ideal process would supply just enough oxygen gas to produce a desired dielectric coating on the substrate without any oxygen gas remaining to contaminate or "poison" the target. Reaching this ideal is difficult because of "hysteresis" behavior.
FIG. 1A is a plot showing the effect of the reactive gas partial pressure on the sputter deposition rate. In this case, the reactive gas is oxygen. The abscissa axis represents the oxygen partial pressure as a percentage of the total gas pressure, with the balance being an inert sputtering gas, typically argon. The ordinate axis represents the sputter yield in arbitrary units. In a pure argon environment (i.e. when the oxygen partial pressure is 0%) the sputter yield is high and a pure metal film is deposited. As the oxygen partial pressure is increased (and argon partial pressure reduced so that the system pressure remains constant), the sputtering target begins to become oxidized. As long as oxygen partial pressure is low enough, the sputter process removes the oxide as quickly as it is formed. The sputter process acts as a pump for the oxygen gas, consuming a large percentage of the available oxygen gas which is incorporated into the coating. The sputter yield will remain approximately constant during this process. This process occurs between points (E) and (A) in FIG. 1A. At some point (A), the oxygen partial pressure will be high enough that the oxide forms on the target surface faster than the process can remove it. At this point, the target develops an oxide skin and the sputter yield decreases dramatically. This process is illustrated in FIG. 1A by the dashed lines showing the transition from point (A) to point (D). When the sputter yield has decreased, the process can no longer use the large amount of oxygen gas which was required when the sputter yield was so much higher. Because the sputter yield is lower, the "pumping efficiency" of the target is greatly reduced. The excess oxygen is partially incorporated into a thicker oxide layer on the target surface, with the balance contributing to an increase of thirty percent or more in total system pressure. Coatings produced under these conditions have dielectric, optically clear properties.
If one begins the process by sputtering a target in a pure oxygen environment (i.e. when the oxygen partial pressure is 100%) the target is oxidized; the sputter yield is low; and the coatings have optically clear, dielectric properties. As the oxygen partial pressure is reduced (and argon partial pressure increased so that the system pressure remains constant), the process remains in the oxidized state until there is an insufficient amount of oxygen gas to replenish the oxide coating of the target which is slowly removed by the sputtering process. This process occurs between points (D) and (C) in FIG. 1A. At some point (C), the target has bare metal exposed and the sputter yield increases back to the level of the metal process. The process is illustrated by the dashed lines showing the transition from point (C) to point (E). As the target consumes the oxygen gas, there is an oxygen deficiency in the system. This deficiency causes even more bare metal to be exposed on the target, which further increases the pumping efficiency of the target. As a result, the system pressure decreases dramatically, falling by thirty percent or more.
The transitions illustrated by the dashed lines in FIG. 1A can occur in less than a second; far too fast for a human operator to adjust the oxygen flow. Ideally, the process should be maintained at some condition between the two transitions, indicated in FIG. 1A by point (B). At this midpoint, the deposition rate is almost as high as that of the metallic process, and the coating properties are close to those of films produced in pure oxygen. The difficulty is that the process at point (B) is highly unstable. The process will move to either point (B') or point (B") under the slightest perturbation. Even if an operator were quick enough to respond to a change, he would not know which way the process was going, oxidized or metallic, until it was too late.
The simplest approach to maintaining the reactive sputtering process within the desired area of the hysteresis region is to increase the pumping of the vacuum system to a degree that the perturbation in the gas consumption due to changes in the target oxidation state is small compared to the amount of gas consumed by the vacuum pump. This has the effect of changing the response curve to that shown in FIG. 1B. The hysteresis is completely eliminated and process control is straightforward. This technique has been used on small research coaters and some production systems but is typically considered too expensive to use with full-size production coaters.
The second approach is to reduce the oxygen partial pressure at the target through the use of baffles, tailored gas introduction systems, etc. The goal of this approach is to maintain an argon-rich environment in the immediate vicinity of the target surface, thus ensuring a high deposition rate. The oxygen is concentrated near the substrate to produce a fully oxidized coating. Sometimes the oxygen is activated with a second plasma source to promote reactivity with sputtered metal. S. Maniv, et al., reported this technique for the deposition of zinc and indium-tin oxides in 1980. (S. Maniv and W. D. Westwood, J. Vac. Sci. Technol. 17,743 (1980)).
A third approach is to closely regulate the amount of oxygen admitted to the process via a closed-loop control system. The process is held at point (B) in FIG. (1A) and as soon as any small perturbation is detected, the control system immediately responds by adjusting the amount of oxygen admitted to the system. The oxygen flow to the target area is regulated based on a measured parameter such as optical emissions or target cathode voltage. An example of this type of system is described in a paper by S. Berg., et al. (S. Berg, H. O. Blom, M. Moradi, C. Nender and T. Larson, Process Modeling of Reactive Sputtering, J. Vac. Sci. Technol. A7(3), 1225 (1989)). In these types of systems, typically precision control of the reactive gas is required to control the process. In order to minimize the response time, the gas valves are located as close as possible to the target, with minimal plumbing between the valve and target. Additional gas tubing after the valve would introduce delays in the gas transport. Sophisticated control valves such as piezo-electric valves, are typically used to provide the fast response time required.
It is desired to have an improved method for controlling the reactive sputtering process within the hysteresis region.